Tuning the Carter Thermoquad Carburetor - TQ Dyno Dial Up!

1/20Dyno operator Tom is ready to demonstrate the improper way to tune a Thermo-Quad carb.

While the AFB and even the AVS carbs get most of the fame, the Carter Thermo-Quad was probably used on more four-barrel Mopar applications than all other carburetors combined. Debuting on the '71 340 (though with a unique metering system in that year) from 1973 until the end of production in 1985, if it was a four-barrel Mopar engine, it had a TQ on top. Interestingly, even among Mopar fanatics, the TQ seems to inspire either wild admiration or open contempt. One thing everyone can agree on is the TQ is king when it comes to flow-per-dollar spent. Basically, in production versions there were two sizes-big and bigger. The small TQ carries 1 3/8-inch primaries, while the larger version measures a substantial 1 1/2-inch. Both share secondary caverns specing out at 2 1/4-inch. That's nearly eight square inches of throttle bore area on the secondary side alone, providing room enough to pass 800 cfm through the smaller version of the carb, and a breezy 850 cfm through the large example.

That's quite a bit of carb, but the TQ seems to defy convention in terms of airflow relative to displacement, since the factory routinely bolted the larger of the two to smog laden 318s. The TQ was hands down the most sophisticated of the Carter four-barrels and the most tunable. For any carb to survive as an OE unit well into the advent of fuel injection and stringent emissions standards, it had to be a precise fuel-metering device. Part of the TQ's secret is in its contoured air door, unlike the flat plate air door used in the AVS, which itself was a quantum leap in technology over the AFB's crude counterweighted velocity valves. The TQ's air door is contoured to create a highly effective venturi in the region of the secondary fuel discharge nozzles. The TQ's sophistication may well have been its undoing as far as universal popularity goes. Just the air door itself can be manipulated by no less than five different settings and adjustments, all of which play a role in how the secondaries react and affect the fuel curve. Add in the tuning permutations possible with three-step metering rods, primary and secondary jets, step-up piston position and tension, accelerator pump timing, stroke and volume, main and auxiliary air bleed circuits, and you can see that the potential to really screw one up is incalculable. Compare that to the most popular carbs, where tuning for most enthusiasts requires just unscrewing a jet and replacing it with one having a bigger number for more gas. Learn the TQ's secrets, however, and these dirt-cheap units can run with the best of 'em.

We recently rebuilt a small TQ for a buddy and wanted to put in some dyno time tuning it to perfection. Glendora Dodge counterman James Schagel was kind enough to donate his time and the use of his Duster for our tuning effort. James' 360-powered A-body was equipped with a conventional high-performance aftermarket square bore carb. Although the object of this study was to delve into the tuning intricacies of the ThermoQuad, we also tuned his carb as part of the deal, which provided us with a mental benchmark of the TQ's performance. Right off the bat, the TQ was performing on par. With both carbs tuned, the TQ actually showed 233 versus 229.7 with the "performance" carb. However, the TQ did have an adapter plate under it that functioned as a carb spacer, adding another variable. One thing is certain, a TQ built and tuned correctly can perform.

TQ Factory Metering Rods and JetsFactory metering rods are all numbered, but the problem is we know of no reference that lists the rod sizes. We have archived a list of the factory rods per the final blueprint specs from Carter Automotive Company. The specs compiled here provide the first comprehensive reference that we know of. These specs give the rod diameter at a glance, to make rod selection and tuning easier. A fatter rod plugs up more of the jet, reducing cross-sectional area, and thus fuel flow. Most TQ rods have three steps: economy, in action when under light throttle cruise; mid, under higher load or throttle opening; and power, high load and WOT. Remember, as soon as the throttle is snapped to wide open, the rod will instantly be up on the power step, no matter the rpm. Primary jets are also numbered, but last three digits of the number relates to the orifice size directly. For instance, a primary jet 4095 is .095-inch or a 4101 measures .101-inch. Similarly, the last three digits of a secondary jet equates to the size as a fraction of an inch. A 5110 jet measures .110-inch, while a 5143 measures .143-inch.